Optical recording medium

ABSTRACT

An optical recording medium having at least one data track and at least one clock track including clock bits provided in a predetermined optical pattern, in which the clock track comprises regular, periodical clock bits arranged regularly in a line and at least one irregular clock bit disposed in the line, breaking the regularity of the periodical clock bits. When a PLL circuit is reinstated into a locked state after it has been unlocked, a leading word can be identified by the irregular clock bit provided in the clock track. This enables data reading thereafter to be accurate.

FIELD OF THE INVENTION

This invention relates to an optical recording medium having at leastone clock track provided along a data track or tracks, in which theclock track has a predetermined optical pattern of clock bits formedthereon.

BACKGROUND ART

Recently, an optical recording medium, which has a data recordingsurface having optically changed patterns, such as light and dark pits,formed thereon to represent digital data, has been attracting publicattention. The optical recording medium could realize a memory of alarge capacity because of its high density data recording capability.Thus, there has been contemplated, as an optical memory, not only a disctype memory but also a card type memory.

In the card type optical memory or optical memory card, recording ismade by forming optical (or magnetooptical) changes in response to datato be recorded at discrete spots on the surface thereof, and then alight beam, such as a laser beam, is projected thereonto to read thechanges.

More specifically, fine unevenness or light and dark patterns areprovided on a surface of a recording medium or card. A light beam isprojected onto such optical patterns to read the recorded data accordingto a difference in reflectivities, refractive indexes, ortransmissivities between the optically changed states of the medium.Alternatively, the recording medium may have photothermo-magneticalrecord thereon and the recorded data may be read depending upon a changeof polarization of the projected light due to the magnetooptical effect.

In juxtaposition with each of the data tracks of the optical memorycard, there are provided a clock track for synchronization in a datawrite/read operation and a tracking guide line as a reference forletting the optical beam from an optical reading system follow the trackaccurately.

The write/read timing for the optical recording medium is conventionallydetermined by a clock signal derived by a detector dedicated fordetecting clock bits of the clock track.

Incidentally, if the clock is not accurately read in reading the clocktrack of the optical recording medium by reason of dust, scratch or thelike on the optical recording medium or fluctuations of the movingvelocity of the recording medium relative to the optical reading system,then it will be difficult to achieve accurate write/read operation. Anabsence of the clock during a short term or in relatively slowfluctuations of the velocity can be coped with by using a phase lockedloop (PLL) circuit which produces a clock of a frequency synchronizedwith the input of the read out clock.

FIG. 5 illustrates a typical arrangement of a prior art PLL circuit. ThePLL circuit 66 comprises a phase comparator 52, a low pass filter (LPF)54, an amplifier 56, a voltage controlled oscillator (VCO) 58 and a 1/Nfrequency divider 53. This circuit compares, in phase, a received inputsignal with an output of the divider 53 so as to match the phase throughcontrol of the VCO 58, thus producing, at the output thereof, a clock ofa frequency of N times of that of the input signal. When N is 1, a VCOoutput fonformed with the frequency of the input signal will appear atthe output terminal of the PLL circuit 66.

However, for a long-term absence of the clock or for an abrupt change ofthe relative moving velocity of the medium, the PLL circuit could notensure the accurate data reading and it may fall into an out-of-lockedstate. In this case, the following problems may occur. When one data isformed of a plurality of words (a "word" used here means one bit or aplurality of bits to be read by one clock), if the PLL circuit getsunlocked or gets into an out-of-locked state, some words will not beread normally. In this state, if the PLL circuit is reinstated into thelocked position again, it can in no way be identified which word in thedata is being read.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the problems involved inthe prior arts and it is an object of the present invention to providean optical recording medium which is capable of identifying a first wordfor a succeeding data when the PLL circuit is reinstated into the lockedposition after the PLL circuit has fallen into the out-of-lockedposition and some recorded words have failed to be read normally.

The present invention features an optical recording medium having atleast one clock track and at least one data track as illustrated in FIG.1, in which said clock track comprises clock bits arranged periodicallyand regularly in a line and at least one irregular clock bit disposed inthe line, breaking the regularity of the periodical arrangement of theclock bits.

The irregular clock bit is provided by forming an optically changedstate same as those of the clock bits at a space between the adjacenttwo clock bits.

The irregular clock bit is provided for a leading word of a data on thedata track.

According to the present invention, the irregular clock bit is disposedin the line wherein the clock bits are arranged regularly, which enablesinformation for the leading word of the data to be read from the opticalrecording medium without lowering the data recording density.

A clock reading circuit for the optical recording medium according tothe present invention preferably comprises, as illustrated in FIG. 2, aphase locked loop (PLL) circuit which receives a read clock obtainedwhen the clock track is read and an irregular clock detection meanswhich compares a clock output of the PLL circuit with the read signaland generates a detection pulse when the irregular clock bit whichbreaks the regularity of the clock bits is detected.

With this arrangement, when the PLL circuit is reinstated into thelocked position after the PLL circuit for the clocks has fallen into theout-of-locked position and the recorded word or words are not readnormally, a first word for one data after the restoration of the PLLcircuit into the locked position can be identified.

The present invention is preferably employed for a system in which theout-of-locked state of the PLL circuit is detected to determine that theclock for data reading is not normal when the out-of-locked state isdetected and inhibit address updating for a storage means which storesthe read data, while allowing only normal data to be stored.

As described above, when the PLL circuit is reinstated into the lockedposition after the PLL circuit has fallen into the out-of-lockedposition, a leading word can be identified by the irregular clock bitprovided in the clock track. Therefore, the read data thereafter can berecognized accurately. This can be attained without lowering therecording capacity of the optical recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing one form of an optical recordingmedium according to the present invention;

FIG. 2 is a block diagram showing a formation of a clock reading circuitfor the optical recording medium according to the present invention;

FIG. 3 is a block diagram of a specific arrangement of the clock readingcircuit for the optical recording medium according to the presentinvention;

FIG. 4 is a timing waveform diagram showing signal waveforms atessential portions of the circuit shown in FIG. 3;

FIG. 5 is a diagram showing an arrangement of a conventional PLLcircuit; and

FIGS. 6 to 8 are explanatory views each showing a modification of theclock track according to the present invention and a signal waveformcorresponding thereto, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described,referring to the attached drawings.

Optical Recording Medium

In an embodiment as illustrated, an optical memory card is contemplatedas an example of an optical recording medium. However, the embodimentmay also be applicable to another type of the optical recording medium.The optical memory card as illustrated is so formed that a plurality ofdata tracks are read simultaneously by each of clock bits. However, thepresent embodiment may also be applicable to the optical memory card inwhich only one data track is read by one clock bit.

The optical memory card to which the present invention is appliedcomprises, as illustrated in FIG. 1, a clock track 30 and a data tracksection 40 including a plurality of data lines 1 to n which are arrangedalong the clock track 30. The clock track 30 is also used as a guideline for tracking in the present embodiment, but it may of course beprovided separately from the guide line.

In the present specification, n bits formed of data lines 1 to n whichare read at one time by one clock bit are defined as one word. The clocktrack 30 consists basically of regular clock bits 32 which are arrangedin line at equal intervals. The clock track 30 further includesirregular clock bits 47 for leading words 46 of the respective data asdescribed above. Each of the irregular clock bits 47 corresponds to theleading word 46 and it is formed to continuously extend in length to asucceeding clock bit. A word 48 may alternatively be used as a leadingword. An interval A between the irregular clock bits is at least of alength of bits in which the PLL circuit can be maintained in a lockedstate.

Clock Reading Circuit

FIG. 3 is a block diagram of one form of a clock reading circuitembodying the present invention. In the circuit as illustrated, a delayelement 67 and a D flip-flop (hereinafter referred to as "D F/F") 68 areused for an irregular clock detecting means of FIG. 2.

A PLL circuit 66 (N=1) receives a read clock signal which is abinary-coded signal changed from a read signal of the clock track 30. DF/F 68 receives the read clock at its D input terminal and it receives,at its clock input terminal, an output 66 from the PLL circuit 66through the delay element 67 to generate a clock output at its Q outputterminal.

In the case where a phase of the output 69 from the PLL circuit 66 isdelayed by a predetermined time from a phase of the read clock, there isno need to provide the delay element 67.

Operation of Clock Reading Circuit

An operation of the circuit shown in FIG. 3 will now be described,referring to a timing diagram of FIG. 4.

The read clock input to the PLL circuit 66 of FIG. 3 has a waveform asshown by (a) in FIG. 4 in which one clock, periodical one, is missing ina term B. The term B corresponds to the irregular clock bit 47 ofFIG. 1. If the PLL circuit 66 is in a locked state, the output from thecircuit 66 is a clock signal of a frequency conformed with a frequencyof the input thereto. This clock signal is utilized as a clock signalfor reading/writing of data.

On the other hand, the clock signal is delayed by the delay element 67to be a delayed clock signal (shown by (b) in FIG. 4. D F/F 68 takes inthe read clock (waveform (a)) by a leading edge of the delayed clocksignal (waveform (b)). The delay time of the delay element 67 is sodetermined that "high" levels of the read clocks may be taken in, exceptwhen the irregular clock bit is being read. Therefore, the Q output(waveform (c)) of D F/F 68 is normally kept "high", while the waveform(b) rises and the Q output of D F/F 68 becomes "low" if the waveform (a)becomes "low" during reading of the irregular clock bit (the term B).

When the waveform (b) rises again thereafter, the waveform (a) isreinstated into the "high" state and therefore the Q output is alsoreinstated into the "high" state. Thus, a detection pulse 70 is outputwhen the irregular clock bit is read. The leading word of the read datais identified in response to the detection pulse.

Other Embodiments

FIGS. 6 to 8 illustrate other forms of the clock track of the opticalrecording medium and waveforms of signals generated from clock readingcircuits which correspond to said other forms of the clock track,respectively. Clock tracks 82, 84 and 86 of FIGS. 6 to 8 have irregularclock bits 47, respectively, which are different from that of FIG. 1.Waveforms (a) of the respective read clocks are also different from thatof FIG. 1 accordingly.

A common feature of the irregular clock bits 47 of the respective clocktracks shown in FIGS. 1, 6 to 8 is that a clock bit is provided at aposition where no clock bit is formed in a clock track comprised only ofregular, periodical clock bits 32. In other words, an optically changedstate identical with those of the clock bits 32 is formed at a spacebetween two clock bits 32 according to necessity. Therefore, althoughthe waveforms (a) are different between FIGS. 6 to 8, the resultantwaveforms (c) are the same. More particularly, since the waveform of theread clock is at a "low" level at rising 72 of the output clockgenerated from the PLL circuit when the irregular bit is read, adetection pulse 70 is then generated.

The present invention is suitably applied to a system in which a readword is stored in a storage means when the PLL circuit is being keptlocked, but the read word is not stored when the PLL circuit is out oflock.

While a few embodiments of the invention have been illustrated anddescribed in detail, various changes and modifications may be introducedwithout departing from the scope of the present invention. For example,the "high" and "low" of the respective waveforms of FIG. 4 may be viceversa.

What is claimed is:
 1. An optical recording medium having a surface provide with a clock track having a plurality of clock bits and a data track section having a plurality of data bits arranged in a line, and the clock bits and data bits being arranged in parallel on said surface so that a plurality of data tracks are read simultaneously by each of the clock bits, and said clock track including irregular clock bits for leading words, at least one leading word being intermediate said line.
 2. An optical recording medium according to claim 1, in which said clock track is a guide line.
 3. An optical recording medium according to claim 1, in which the clock track consists of regular clock bits between said irregular clock bits arranged in a line at equal intervals. 